weird ESXi host: some VMs in it unpingable : narrowed to vmnic

I have 5 x3650 M3 (let's call them s4, s5, s6, s7, s8) all of the same hardware
specs in one cluster : all are on ESXi 5.0 Upd 1.

Each & everyone of the ESXi host's vmnic are the same ie:
vmnic0 ==> Management VLAN
vmnic1 ==> vMotion
Quad NIC 1 has vmnic4,5 connected for Prod VLANs
Quad NIC 2 has vmnic8,9 connected for the same Prod VLANs

They have been running fine since mid last year till about
1 month back when 1-3 VMs in s4 would suddenly become
unpingable (as reported by Tivoli & I would try to Rdp into
them upon getting Tivoli alert but can't access).

Troubleshooting done so far:
a) vCenter could still console into all the VMs (affected &
unaffected VMs) in s4 but the affected VMs can't ping
to their gateway IP address though "ipconfig" still show
the IP addresses of the affected VMs & their respective
gateway IP addressses. The affected VMs in s4 could ping
other VMs in s4 that are of the same VLAN/subnet but
not other VMs of different VLANs/subnets in s4. Affected
VMs also can't ping other VMs of same VLAN that sit
inside s5-s8

b) while inside the affected VMs console, I noticed
under Win 2008 R2 Standard x64 the affected
NIC shows as "Unidentified network". For those
VMs not affected still in s4, it shows as "corp.local"

c) from vCenter's "Edit Settings" deleted the NIC
adapter & recreate back : still the same issue

d) reboot the affected VMs & after they've booted
up (& still stay in s4), still no joy

e) the moment I vMotioned out the affected VMs
to another host (s5 - s8), the VMs became
pingable again

f) from vCenter, selected the vSwitch, "Managed Hosts",
check to select s4 & then we disabled vmnic4/5, all VMs in
s4 (including those VMs that were pingable so far while in s4)
immediately became unpingable. Then we enable back
vmnic4/5 & disabled vmnic8/9 (the other pair of NIC ports
on the other QUAD NIC), all VMs in S4 became pingable
again. Got IBM to replace this 'suspected' Quad NIC but
still no joy.
On the pair of stacked Cisco C3750 switches that vmnic8/9
are connected, it showed high packet drops with 0 payload
(ie input rate = output rate = 0 kbps)

g) All LEDs on the pair of switches are green & all LEDs on
s4's NICs are green

h) I transferred the cable of vmnic8 to a free port vmnic1
(the onboard NIC), then used vCenter to disable vmnic4+
5+8+9 but enable vmnic1 ("Managed Hosts") & all VMs
in s4 became unpingable. I swapped this piece of cable
with a tested working cable & still no joy

>I would swap working ports for "non-working ports" as a check if the issue
> is physical switch or server.

Had done the swapping & isolated that it's due to both the Cisco switches'
(a pair of C3750X-48 stacked together) ports issue: got the network engr
to provision 2 other ports gi1/0/12 & gi2/0/12 on the same pair of
switches & vmnic8/9 now worked ==> verified by disconnecting all ports
& connecting up vmnic8 only to gi1/0/12 & then disconnect it & connect
up only vmnic9 to gi2/0/12 & all VMs in s4 are pingable.

Just one last question:
with 2 ports working & another 2 ports not working, shouldn't VMware
reroute all traffic to the 2 working ports (ie vmnic4 & vmnic5) ? This
is an LACP dot1q trunk of the four ports vmnic4/5/8/9 so I'm expecting
that with Cisco Cdp being used (as shown in vCenter), ESXi should be
smart enough to route all traffic to the 2 remaining useable ports,
shouldn't it?

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